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SIMULATION OF VORTEX SOUND USING THE VISCOUS/ACOUSTICSPLITTING APPROACHTing H.Zheng1,Shiu K.Tang2,Wen Z.Shen31Department of Applied Mechanics,Sichuan University,Chengdu,P.R.China,6100652Department of Building Service Engineering,The Hong Kong Polytechnic University,Hong Kong,P.R.China3Department of Engineering,Technical University of Denmark,DenmarkE-mail:besktang@polyu.edu.hkReceived March 2010,Accepted April 2010No.10-CSME-14,E.I.C.Accession 3177ABSTRACTA numerical viscous/acoustic splitting approach for the calculation of an acoustic field isapplied to study the sound generation by a pair of spinning vortices and by the unsteadyinteraction between an inviscid vortex and a finite length flexible boundary.Based on theunsteady hydrodynamic information from the known incompressible flow field,the perturbedcompressible acoustic terms are calculated and compared with analytical solutions.Resultssuggest that the present numerical approach produces results which are in good agreement withthe analytical solutions.The present investigation verifies the applicability of the viscous/acoustic approach to flow structure-acoustic interaction.Keywords:computational aeroacoustics;vortex sound;viscous/acoustic approach.SIMULATIONS NUME´RIQUES DE PRODUCTION SONORE PAR DESE´COULEMENTS TOURBILLONNAIRES A`L’AIDE DE DE´COMPOSITIONVORTICITE/POTENTIELRE´SUME´Une me´thode de de´composition des e´coulements en composantes acoustiques et vorticellesest applique´e au proble`me de la ge´ne´ration sonore pour un couple de deux vortex tournants,ainsi que le bruit d’interaction entre un vortex ide´al et une surface flexible.Les contributionsline´aires acoustiques sont e´value´es par comparaison avec la solution analytique connue pourl’e´coulement ide´al potentiel incompressible.Le champ sonore re´sultant est compare´ a`unesolution analytique.Les re´sultats sont satisfaisants,et sugge`rent des applications possibles pourles interactions fluides-structures.Mots-cle´s:calcul ae´roacoustique,son tourbillon,me´thode de de´composition des e´coulements.Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 391.INTRODUCTIONAeroacoustics,which is the science of noise generation by airflows and its propagation,is arelatively young discipline compared to other more classical fields of mechanics.Typically inaeroacoustics,the solutions can be grouped into the frequency-domain solution and the naturalvariables (x,t) solution.Within the latter,the different approaches in the analysis can becategorized into three groups.The first group makes use of the acoustic analogy.The most renowned acoustic analogy isdue to Lighthill [1].He rearranged the mass and momentum equations to obtain aninhomogeneous wave equation which describes the generation and propagation of an acousticwave in a medium at rest.Ffowcs Williams and Hawkings [2] generalized the Lighthill’sacoustic analogy to include the effects of surfaces in arbitrary motions.Powell [3] and Mo¨ hring[4] put forward the vortex sound theory for predicting the sound from two and three-dimensional compact vortical flows.This group of approach can compute the noise directivityin an economical way because the flow in the far field is actually uniform.However,all of theacoustic analogies are based on a variety of assumptions such as compact source and low Machnumber.Acoustically compact sources (including a vortex) mean that the size of the acousticsource is much smaller than the wavelength of the acoustic waves generated.In fact,in manypractical cases,the latter is comparable to the former.The advantage of numerical simulation isthat this compact source assumption is not needed.The second approach makes use of direct numerical simulation (DNS),where both the fluidmotion and the generated sound are directly computed by means of the Navier-Stokesequations.One of the advantages of DNS is its capability to compute the generation andpropagation processes without suffering from restrictions such as low Mach number,highReynolds number and compactness of the source region in principle.DNS methods arespecially suited to model broadband noise generated by turbulence.However,it is difficult todistinguish pressure fluctuations from the sound generated as the acoustic perturbations aretypically at least 10 times weaker than the corresponding hydrodynamic perturbations [5].Also,DNS requires tremendous computational resources especially when the flow Mach number islow and it is difficult to compute propagation over long distances.Therefore,this approach ispreferable for the study of aeroacoustics at a relatively higher Mach number condition [6].Someauthors neglect the viscosity and flow turbulence and solve the Euler equations for short-time,high-speed sound-generating phenomena such as shock wave-vortex interaction (for instance,Inoue and Hatakeyama [7]).The third approach uses the hydrodynamic/acoustic splitting method which decomposed theflow field into incompressible hydrodynamic and compressible perturbation equations (forinstance,Hardin and Pope [8]).This splitting method has further been modified by Shen andSørensen [9].Bogey et al.[10] computed the sound radiated by unsteady fluid motions using anacoustic analogy based on the linear Euler equations forced by aerodynamic source terms.Seoand Moon [11] revised this hydrodynamic/acoustic splitting method through a consideration ofthe perturbed vorticity transport equation.More recently,Ewert and Schro¨ der [12] derivedseveral source term formulations to allow the acoustic simulation based on incompressible andcompressible flow solutions.This approach does not allow acoustic backscattering into the flowsolution.It makes possible the computation of aeroacoustic noise generation and propagationin non-uniform unsteady viscous flows in complex domains.This poses substantial advantageover the first approach.Also,compared with the acoustic analogy theories,the sound strengthis obtained directly in this approach so that both sound radiation and scattering can beTransactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 40accounted for simultaneously [6].On the other hand,the computing cost required is much lowerthan that for the DNS solution of the time-dependent subsonic flow field.The turbulent air flow inside air conditioning ductwork induces pressure fluctuations on theduct walls,resulting in the vibration of the latter which in turn radiates sound to both the ductinterior and the external environment.Understanding the flow structure-acoustics interactiontherefore is important for improved duct noise control design.Tang et al.[13] developed atheoretical model to investigate the sound generated by the unsteady interactions between avortex and a finite length flexible boundary in an otherwise rigid wall at low Mach numbers.Itis concluded that the time fluctuating volumetric flow rate induced by the flexible boundaryvibration and the vortex acceleration are two major sources of sound.However,real duct flows are turbulent and analytical solution is hard to find for this kind ofaeroacoustics problem while numerical solution will make it possible to investigate theturbulence-structure interaction.A turbulence modeling technique can be used to recover theturbulent activities and therefore provide the hydrodynamic terms needed in the acoustic/viscoussplitting method.The acoustic radiation can then be obtained directly through numerical means.In this paper,the viscous/acoustic splitting method is applied to calculate the soundgeneration by the spinning vortices [14–15] and by the vortex induced flexible wall vibration[13].The computed results are compared with the analytical solutions and the performance ofthis viscous/acoustic splitting method is examined.The major aim of this paper is to verify theapplicability of the numerical technique to the simplified models of flow noise radiation withand without a nearby solid structure.It is hoped that the present results will provide clues forthe modeling of more complicated aeroacoustic problems.2.NUMERICAL PROCEDURES2.1.General FormulationsAccording to Shen and Sørensen [5],the compressible solution can be decomposed as:u~Uzu0,v~Vzv0,p~Pzp0,r~rozr0,ð1Þwhere U,V,P and roare the background mean flow longitudinal and transverse velocities,fluidpressure and density respectively.They can be obtained analytically or numerically using low-order schemes of computational fluid dynamics,while u9,v9,p9 and r9 are correspondingacoustic disturbances obtained from the numerical solutions using high-order schemes in spaceand time to precisely capture the sound pressure.Substituting the above equations into the compressible Navier-Stokes equations,neglectingthe viscous terms and the higher order perturbations,and subtracting the incompressibleconservation equations,the set of governing equations for the two-dimensional acoustic fieldscan be expressed as:Lr0LtzLfiLxi~0LfiLtzLLxjp’dijzfiUjzu0j

zroUiu0jh i~0Lp0Lt{c2Lr0Lt~{LPLt:ð2ÞTransactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 41wherefi~ru0izr0Ui,ð3Þc,the ambient speed of sound,is obtained from the equation c2~cp=r~c Pzp0ð Þ=rozr0ð Þ andc is the specific heat ratio.Details of the formulation of Eq.(2) are given in the Appendix.Itshould be noted that the only acoustic source coming from the incompressible solution is theinstantaneous pressure and the acoustic calculations can thus be started at any time during theincompressible computation.The initial conditions arer0~0,u0i~0 and p0~po{P,ð4Þwhere pois the ambient air pressure.2.2.Numerical DiscretizationThe acoustic waves are non-dispersive and non-dissipative during their propagation.They areparticularly susceptible to numerical dispersion and dissipation.Numerical dispersion distortsthe phase between various waves and numerical dissipation reduces the gradients in thesolutions [16].One straightforward way to overcome these problems is to use a very fine gridtogether with a standard low-order method.However,this approach is not truly feasiblebecause of the dramatic increase of the computation time and memory.To accurately resolvethe propagation of acoustic waves,finite difference and time-marching schemes that have lownumerical dissipation and can accurately present the dispersion relation for the inviscidequations are required [7].In this study,the forth-order-accurate central-difference compact scheme [17] that has lowdissipation and near spectral representation of the dispersion relationship is chosen for thenumerical approximation of the spatial derivatives in Eq.(2):14V’i{1zV’iz14V’iz1~34DxViz1{Vi{1ð Þ,ð5Þwhere V~r0,f10,f20,p0,and Dx is the grid size.At the boundary of computational domain,athird-order-accurate compact scheme biased toward the interior nodes is used [17]:V’1z2V’2~1Dx{52V1z2V2z32V3

,V’nz2V’n{1~1Dx{52Vnz2Vn{1z32Vn{2

ð6ÞThe centered nondissipative compact scheme is desirable for the computation of linear wavepropagation.However,the inherent lack of numerical dissipation may also result in spuriousnumerical oscillations and instability.To overcome its unrestricted growth of spuriousperturbations,the tenth order filter scheme [15] is employed as such scheme does not amplifyany waves,preserves constant functions and completely eliminates the odd-even mode whenuniform meshes are taken.Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 42If a solution vector is denoted by l,the filtered values^ll satisfy,af^lli{1z^llizaf^lliz1~XNn~0an2liznzli{nð Þ,ð7Þwhere afis the filter coefficient and {0:5vafƒ0:5.A higher value of afcorresponds to a lessdissipative filter and there will be no filtering effect for af~0:5.In this paper,af~0:45.TheNz1 coefficients a0,a1, aN,are functions of af.With a proper choice of the coefficients,Eq.(7) provides a 2Nth-order formula on a 2Nz1 point stencil.In this study,the tenth orderfiltering scheme are employed and the corresponding coefficients of ancan be found in Visbaland Gaitonde [18].Since the compact schemes near boundaries are not symmetrical and they contain dissipationfactors already,no filtering scheme is needed at the boundary point 0 and IL – 1.At a near-boundary point,i,the one-sided filter formula is given byaf^lli{1z^llizaf^lliz1~X10n~0an,iln,i[ 1, 4f gð8Þandaf^lli{1z^llizaf^lliz1~X10n~0aIL{n,i^llIL{n,i[ IL{5, IL{1f gð9ÞMore information about the coefficients of ancan be found in Gaitonde and Visbal [19].The explicit fourth-order Runge-kutta time advancement scheme proposed by Williamson[20] and implemented by Wilson et al.[21] is adopted in the present study as it gives lowamplitude and phase errors of traveling wave solutions.To simplify the discussion,consider thefollowing convective wave equation:LVLtzLVLxzLVLy~0:ð10ÞThe equation is advanced from time level n to n+1 in Q sub-stages.The advancement fromsub-stage M to M+1 is defined byVMz1~VMzbMz1HMDt,ð11Þwhere M51,…,5 is the particular stage number,Dt is the time step,bMis a coefficient that canbe found in Williamson [20] and VMrepresents the V value at the Mthsub-stage.HMis the sumof all the right-hand-side terms in Eq.(2).The accumulation from the previous sub-stage orfrom the initial conditions at t 5 0 isHM~LVMLxzLVMLyzaMHM{1,ð12ÞTransactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 43where aMis another coefficient that can be found in Williamson [20].The low-storagerequirement is accomplished by continuously overwriting the storage location for the timederivatives and unknown variables at each sub-stage:aMHM{1?HMand VMzbMz1HMDt?VMz1ð13Þwhere Rindicates that the storage locations HM-1and VMare overwritten by HMand VM+1ateach time step respectively.2.3.Boundary ConditionsAeraocoustic problems are usually defined on an infinite or semi-infinite domain.Thenumerical solution of all the discrete equations requires truncation of the computationaldomain and the imposition of artificial numerical boundary conditions at the truncated domainboundaries.These artificial boundaries must not only ensure non-reflection of waves,but alsoaccount for the direction of mean flow with respect to the boundary.Non-reflectingcharacteristic boundary conditions based on Thompson’s technique [22] are used because theyare straightforward and easy to apply.There is no obvious reflection observed in the presentcomputations.For the case of flexible wall discussed later,the vibration amplitude of the flexible boundaryis assumed small and this boundary is modeled as a rigid surface with distributed time varyingnormal velocity perturbations [13].3.ILLUSTRATIVE EXAMPLES3.1.The Spinning Vortex PairThe sound generated by a spinning vortex pair has an analytical solution [14] and has beeninvestigated numerically by Lee and Koo [15].This serves as a simple validation case of thepresent computation code.The two identical rectilinear vortices,each of a circulation C and are separated by a distance2ro,undergo co-rotational motions along a circular path with radius ro(Fig.1).The period ofsuch motion is T~8p2r2o

C,the angular speed v~C

4p2r2o

and the rotational Mach numberMr~C=(4proc).The inviscid incompressible flow solution can be expressed in term of apotential function w(xo,yo) [23]:w~C2ptan{1y{yox{xo

ztan{1yzyoxzxo

,ð14Þwhere (xo,yo) is the vortex location.The hydrodynamic velocity and the hydrodynamic pressureP can be obtained by differentiating Eq.(3) with respect to (x,y) and t respectively:U~LwLx,V~LwLyand P~po{roLLtw xo,yo,tð Þ{12r U2zV2

:ð15ÞFrom the asymptotic expansions by Muller and Obermeier [14],the analytical acousticpressure variation produced by a spinning vortex pair at a distance r from the vortex systemcentroid,which is (0,0) in this case,isTransactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 44p0rð Þ~roC464p3r4oc2H(2)2krð Þ:ð16ÞComputations are performed in the domain {20vxv20½ |{20vyv20½ for C/(cro) 5 1.0and Mr50.0796 (and thus k 50.1592).The 300|300½ grids are used.The Vatistas vortex-coremodel24is adopted in order to avoid the singularity at the center of the vortex.At a distance sfrom the vortex center,the tangential fluid velocity vhis given by:vh~C2pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃR4czs4p,ð17Þwhere Rcis the core radius.Figures 2a and 2b present the calculated acoustic pressure contours for the unfiltered andfiltered solutions respectively at t 5 5 when the waves are still sufficiently far away from thecomputational farfield boundaries.A double-spiral pattern of a rotating quadrupole can beobserved.The filter is applied to the conserved variables and sequentially in each coordinatedirection after each sub-iteration of the implicit Runge-Kutta scheme.The results obtainedwithout a filter show the appearance of high-frequency oscillation and instability as the wavepropagates outward in Fig.2(a).When the high-order filter is employed,these unwantedoscillations are completely eliminated.Figure 2c illustrates the acoustic pressure contours att 5 20 (filter applied).A comparison of the acoustic pressure profile at this moment along aradial line from the center to the lower-right corner of the computational domain with thetheoretical result is shown in Fig.3.Very good agreement has been reached between thenumerical results and the analytical solution.Fig.1.The spinning vortex pair.– – – –:Path of vortex motion;N:vortex of circulation C (positive).Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 45Fig.2.Rotating quadrupole produced by the spinning vortex pair (a) t 55 without filter;(b) t 55 withfilter;(c) t 520 with filter.–––:Positive contour;– – – –:negative contour.Contour levels:¡2.561025;¡561025;¡7.561025;¡161024;¡1.2561024;¡1.561024;¡1.7561024;¡261024.Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 463.2.Vortex Induced Flexible Wall VibrationFigure 4 illustrates the schematic of the vortex induced flexible wall vibration model,which isan extract from Tang et al.[13].A vortex of strength (circulation) C initially located farupstream of a flexible boundary of length L at a distance d above a rigid plane is considered.The flexible boundary is at rest initially.The vortex motion creates an unsteady fluid pressureon this boundary,causing it to vibrate and this vibration eventually gives rise to a fluctuatingvelocity field,which affects the motion of the vortex.As stated in Tang et al.[13],the vibration amplitude g of the flexible boundary is assumedsmall compared to the distance from the vortex center to the plane and this boundary ismodeled as a rigid surface with distributed fluctuating velocity v x,tð Þ.Fig.3.Radial variation of acoustic pressure along a line making 245u with the x-axis at t 520.–––:Present result;–?–:analytical solution.[17]Fig.4.Schematic of the vortex induced wall vibration model (from Tang et al.[13]).Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 47The potential function at the position (x,y) in the flow field is a combination of the vortexpotential and the flexible boundary vibration potential13:w~C2ptan{1y{yox{xo

{tan{1yzyox{xo

z1pðL=2{L=2v logﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃx{xvð Þ2z y{gð Þ2qdxv,ð18Þwhere (xo,yo) is the time varying vortex position and (xv,g) represents the location of a point onthe flexible boundary.The induced velocity at the vortex position (xo,yo) due to the boundaryvibration,Vi,can be determined using potential theory [25] asVi~^xxpðL=2{L=2v xo{xð Þxo{xð Þ2zyo2dxz^yypðL=2{L=2vyoxo{xð Þ2zyo2dx ð19Þwhere the caret denotes unit vector in the direction indicated.The vortex velocity V,can thus beapproximated as:V~C4pyo^xxzVið20ÞThe vortex movement path and the movement of the vibrating flexible boundary are obtainedfrom the results of Tang et al.[13] with an in vacuo wave speed along the flexible boundaryequals twice the initial speed of the vortex and a normalized damping coefficient of unity.Thecorrespond Mach number of the initial vortex speed M equals 0.05.The vortex is initiallylocated at (24,1).Figure 5 illustrates the flight path of the vortex center.The vortex initiallymoves parallel to the wall.It then moves transversely towards the flexible wall when it is in theFig.5.Vortex flight path for M 5 0.05 (from Tang et al.[13]).Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 48proximity of the flexible wall.It resumes its original height afterwards.The initial vortexvelocity is normalized to 1.As described in the spinning vortex case,the hydrodynamic velocity can be obtained by therelationship between velocity and the potential function,while the hydrodynamic pressure canbe calculated by using the unsteady Bernoulli’s equation.The induced longitudinal andtransverse velocities at a position (x,y) due to the boundary vibration are respectively:Uflex~1pðL=2{L=2v x{xvð Þx{xvð Þ2zy2dxvand Vflex~1pðL=2{L=2vyx{xvð Þ2zy2dxv:ð21ÞThose due to the vortex areUvor~C2px{xox{xoð Þ2z y{yoð Þ2{x{xox{xoð Þ2z yzyoð Þ2"#ð22aÞand Vvor~C2py{yox{xoð Þ2z y{yoð Þ2{yzyox{xoð Þ2z yzyoð Þ2"#:ð22bÞThe corresponding hydrodynamic pressure related to the flowfield induced by the vortex andthe flexible wall vibration is respectively:Fig.6.Effects of grid refinement on the computed acoustic pressure.Test position:(40,40).–?–:1006100;–?–:2006200;– – – –:3006300;–––:4006400.Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 49Pvor~po{roLwvorLt{12r U2voirzV2vor

and Pflex~po{roLwflexLt{12r U2flexzV2flex

:ð23ÞThe acoustic field induced by the moving vortex can then be obtained by substituting itsincompressible hydrodynamic terms into the acoustic disturbance equation,Eq.(2),and so doesthat from the flexible wall vibration.Agrid refinement study is conducted using the Vatistas vortex model [23].The computationaldomain S~{20vMxv20½ |0vMyv40½ is covered with 1006100,2006200,3006300 and4006400 uniform numerical grids.The acoustic pressure time history at the point (40,40) ispresented in Fig.6.For the coarse grid of 1006100,there is an obvious difference between thecorresponding results and those of the 4006400 grid.The acoustic pressure profiles for the3006300 and 4006400 grids almost overlap with each other and the maximum difference isbelow 1%.Therefore,the 3006300 uniform numerical grid is used in this study.The time stepDt is 0.01 in consistence with that adopted by Tang et al.[1].The tenth order filter scheme isagain applied.Fig.7.Sound fields generated during vortex-flexible boundary interaction at t – to5 9.M5 0.05.(a) By flexible boundary vibration;(b) by unsteady vortex motion.Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 50Figure 7 illustrates the acoustic pressure patterns at t – to59 when the vortex has propagatedfar downstream of the flexible wall boundary to x,8,where todenotes the time at which thevortex passes across the plane x 5 0.The vibration of the flexible wall creates a monopole asFig.8.Acoustic pressure radiation contours.M50.05.(a) t – to51 (vortex near to the leading edgeof flexible boundary);(b) t – to5 5 (vortex at trailing edge of flexible boundary);(c) t – to5 9(vortex far downstream of flexible boundary at x,8).Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 51shown in Fig.7(a) and is still significant even when the vortex is far away fromthe flexible wall.The unsteady vortex motion generates a dipole but the duration of its generation is relativelyshort Fig.7(b).This dipole is generated when the vortex is near to the flexible wall ( Mxj jƒ0:05with My0*0:05),such that the centre of the radiation appears very near to the origin of theco-ordinate system.The amplitude of the vibration monopole is approximately five times thatof the vortex dipole.All these agree with the theoretical calculations of Tang et al.[13].Figure 8 summarizes the development of the overall sound field.At t – to51 when the vortexis close to the x 5 0 plane (centerline of the flexible wall),a strongly asymmetric pressureperturbation pattern can be found somewhere near to the flexible wall.This is also the time atwhich intense interaction between the vortex and the flexible wall is taking place.The verysymmetrical radiation in the outer area is fromthe small flexible wall vibration when the vortexis moving gradually towards the flexible wall.The vortex dipole is therefore significant at thetime when the vortex is moving over the flexible wall.Figure 8(b) suggests that the dipole lastsfor only around two to three cycles.The whole sound field is eventually dominated by thecontinuous monopole from the wall vibration Fig.8(c).One can also observe from Fig.8 thatthe radiation directivity is biased to the downstream when the vortex is above the flexible wall,which is also the instant of high transverse vortex acceleration or deceleration.This is also in-line with the theoretical deduction of Tang et al.[13].Figure 9 illustrates the acoustic pressure variation along the h 545oradial line at t – to512.Fairly good agreement between the theoretical results of Tang et al.[13] and the presentsimulation is observed at large radius.It should be noted that the results of Tang et al.[13] arefor the far field only and thus discrepancy between the present computed results and that ofTang et al.[13] at small radial distance can be expected.Similar observations can also be madeat later instants and along other radial lines and thus are not presented.Similar results areobtained with other combinations of vortex system parameters studied by Tang et al.[13] andFig.9.Radial variation of acoustic pressure along h 5 45u.For M 5 0.05 at t – to5 12:–––:present result;–?–:analytical solution [13];for M50.1 at t – to520:– – –:present result;–??–:analytical solution [13].Transactions of the Canadian Society for Mechanical Engineering,Vol.35,No.1,2011 52thus they are not presented.The agreement between computed and analytical results in the farfield for M 5 0.1 is also very satisfactory.4.CONCLUSIONSAcomputational aeroacoustic technique,which splits Euler equations into the hydrodynamicterms and the perturbed acoustic terms is applied.First,the case of the sound generation due toa spinning vortex pair is taken as a benchmark testing case to valid the code.A very goodagreement between the numerical simulation and the analytical results is observed.This method is then applied to the case of sound radiation due to the unsteady interactionbetween an inviscid vortex and a finite length flexible boundary on an otherwise rigid horizontalplane.Based on the unsteady hydrodynamic information from the known incompressible flowfield,the perturbed compressible acoustic terms are calculated.Calculated results are comparedwith analytical solutions obtained by the method of matched asymptotic expansions.Goodagreement between the simulation and the semi-analytical results of Tang et al.[13] is againobserved.The present investigation verifies the applicability of the viscous/acoustics splittingapproach to flow structure-acoustic interaction.It also implies the possibility of extending thecurrent research to study the complicated interaction between flow turbulence and sound insidea duct silencer for the future development of low self-noise efficient silencing devices.AcknowledgmentsThis study is mainly supported by a grant from Research Grant Council,The Hong KongSpecial Administration Region Government,Hong Kong,China (Project no.PolyU5266/05E).The minor financial support from the Research Committee,The Hong Kong PolytechnicUniversity (Project no.G-YD59) is also appreciated.REFERENCES1.Lighthill,M.J.,‘‘On sound generation aerodynamically I.General theory,’’ Proceedings of theRoyal Society A,Vol.211,pp.564–587,1952.2.Ffowcs Williams,J.E.,Hawkings,D.L.,‘‘Sound generated by turbulence and surfaces inarbitrary motion,’’ Philosophical Transactions of the Royal Society of London Series A,Vol.264,pp.321–342,1969.3.Powell,A.,‘‘Theory of vortex sound,’’ Jounal of the Acoustical Society of America,Vol.36,pp.177–195,1964,4.Mo¨ hring,W.,‘‘On vortex sound at low Mach number,’’ Journal of Fluid Mechanics,Vol.85,pp.685–691,1978.5.Shen,W.Z.,Sørensen,J.N.,‘‘Aeroacoustic modeling of low-speed flows,’’ Theoretical andComputational Fluid Dynamics,Vol.13,pp.271–289,1999.6.Shen,W.Z.,Michelsen,J.A.,Sørensen,J.N.,‘‘A collocated grid finite volume method foraeroacoustic computations of low-speed flows,’’ Journal of Computational 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Lxj~0LpLt~c2LrLt:ðA1ÞDecomposing the compressible solution as the sum of a mean and an acoustic disturbance:ui~Uizu0i,p~Pzp0,r~rozr0,ðA2Þit can be shown by substituting (A-1) into (A-2) thatL r0zroð ÞLtzL r0zroð Þ Uizu0i